1. Field of the Invention
The invention relates to a color image formation apparatus which forms a synthetic image by superimposing image information by utilization of the electrophotography technique.
2. Description of the Related Art
In an image formation apparatus adopting the electrophotography technique, an electrophotographic photosensitive member serving as an image carrier is charged by a charger, and the photosensitive member is exposed to radiation in accordance with image information, to thereby form a latent image. A toner image into which the latent image is developed by a developer is transferred onto a sheet material or the like, to thereby produce an image.
In association with colorization of an image, a plurality of pieces of tandem image formation apparatus have hitherto been proposed. Such an apparatus is provided with a plurality of image carriers to be subjected to such a round of image formation processes. Color images; i.e., a cyan image, a magenta image, a yellow image, and preferably a black image, are formed on corresponding image carriers. Respective color images are transferred onto sheet material at respective transfer positions of the respective image carriers in a superimposed manner, thereby producing a full-color image. Such a tandem multiple-image formation apparatus is said to be advantageous in achieving speedup, because image formation sections are provided for respective colors.
As described in, e.g., Japanese Patent Publication No. 2000-284592, a tandem multiple-color image formation apparatus has an image formation unit for forming toner images of yellow (Y), magenta (M), cyan (C), and black (K) colors; an exposure device for forming an electrostatic latent image by outputting an image signal; an intermediate transfer belt which is made in the form of a closed loop and travels; and a fuser. A toner image formed on the intermediate transfer belt is transferred onto paper P supplied from a paper cassette, and the toner image is fixed on the paper by means of the fuser.
The yellow (Y) image formation unit, the magenta (M) image formation unit, the cyan (c) image formation unit, and the black (K) image formation unit share a common structure. One of the image formation units is shown in FIG. 9, which is a schematic front view of the principal section showing an example configuration of a related-art development unit and that of a related-art photosensitive unit. As illustrated, the image formation unit is a combination of a photosensitive unit and a development unit. The photosensitive unit has: a photosensitive drum 101, on a circumferential surface of which an electrostatic latent image is to be made by means of a laser beam emitted from an exposure device (not shown); a charger roller 102 for charging the latent image; and a cleaning blade 103. The development unit includes a toner feed tank 104, a toner feed roller 105, and a developer roller 106 to be brought into contact with the photosensitive drum 101, and a toner thin-layer blade 107 which comes into contact with the developer roller 106, to thereby make a toner layer a uniform thin layer, and which charges the layer with a predetermined potential.
In such an image formation unit, an electrostatic latent image of image information is formed on the photosensitive drum 101 by means of the exposure device. The electrostatic latent image is made visible in the form of a toner image by the developer roller 106 and transferred onto the intermediate transfer belt. In a four-color image formation apparatus, toner images of respective colors are transferred onto the intermediate transfer belt, to thereby finally produce a multiple color image.
However, as illustrated, the cleaning blade 103, the developer roller 106, and the toner feed roller 105 are arranged in this order with the photo sensitive drum 101 interposed between the blade 103 and the developer roller 106. Hence, the intermediate transfer belt in one image formation unit becomes longer in the direction in which the belt is to travel (i.e., a horizontal direction in the drawing). For this reason, an interval between the photosensitive drums 101 of the four-color image formation units tend to become greater.
As mentioned above, if the interval between the photosensitive drums 101 is excessive, a limitation is imposed on miniaturization of the apparatus. Further, an interval between transfer positions on the intermediate transfer belt also becomes wider. In the case of formation of a multiple color images, the transfer accuracy of color is deteriorated, thereby inducing color misregistration and resulting in a failure to form a superior color image. In addition, the transfer belt must be made longer, and hence a color image becomes apt to color misregistration in the same manner, thus presenting a problem of the apparatus becoming bulky and expensive.